Cosmology and the X-ray source count.

A complete sample of extragalactic X-ray sources is used to predict the form of the source counts at deep levels. In principle, cosmological effects are observable at source intensities of about 0.1 Ariel count/s, only slightly below the level of present observations. Uncertainties in the extrapolated source counts arising from the choice of cosmological model and the incompleteness of the available observational data are considered. Given sufficiently complete data for a sample of a few hundred bright X-ray sources, the deep-source counts could constitute a useful confirmation of the conventional Friedmann-Robertson-Walker world models.